Wireless devices have become an integral part of society as data tracking and mobile communications have been incorporated into a wide variety of products and practices. For example, radiofrequency identification (RFID) systems are commonly used to track and identify objects such as products being shipped, vehicles passing through transit points, inventory in a warehouse or on an assembly line, and even animals and people via RFID trackers that are implanted or worn. Internet of Things (IoT) is another area in which wireless devices are used, where networked devices are connected together to communicate information to each other. Examples of IoT applications include smart appliances, smart homes, voice-controlled assistants, wearable technologies, and monitoring systems such as for security, energy and the environment.
Since many applications require these wireless electronic devices to be very small and portable, thereby limiting the manner in which the devices can be electrically powered, energy harvesting (EH) is often utilized as an additional energy source for the devices. Energy harvesting is generally a process by which energy is derived by an energy harvesting component or device from a variety of energy sources that radiate or broadcast energy intentionally, naturally, or as a byproduct or side effect. Types of energy that can be harvested include electromagnetic (EM) energy, solar energy, thermal energy, wind energy, salinity gradients, and kinetic energy, among others. For example, temperature gradients occur in a region surrounding an operating combustion engine. In urban areas there is a large amount of EM energy in the environment because of radio and television broadcasting. Energy harvesting circuits or devices can thus be placed in, on or near these regions or environments to take advantage of the presence of these energy sources, even though the energy level from these types of energy sources may be highly variable or unreliable. For instance, antennas can be used to capture radiofrequency (RF) energy from EM sources such as cell phones, WiFi networks, and televisions. Energy harvesting is generally distinguished from a direct supply of energy provided through dedicated hardwired power transmission lines, such as that provided by an electrical power utility company through a power grid to specific customers, each of which is an added power load for the energy source.
In some situations, the energy available for harvesting is also known as background, ambient or scavenged energy that is not specifically intended to be transmitted to any particular customer or receiver for the purpose of powering a receiving device. An example of background or ambient energy is the natural EM radiation emitted as an unavoidable side effect or byproduct of many types of electrical devices or transmission lines. Radio frequency broadcasts from ground, air or satellite radio transmitters, in contrast, may be intended to be used by a receiver for telecommunication purposes, but that radio frequency energy (which is EM radiation) is also capable of being used for unintended energy harvesting purposes. In these “unintentional” situations, the energy harvesting circuit simply intercepts the ambient energy whenever or wherever it is available, without being an added power load for the energy source. In other situations, a dedicated wireless EM energy transmitter can be provided to broadcast or beam EM radiation where energy harvesting circuits or devices are known to be present for intentional harvesting or capturing by the energy harvesting circuits or devices, thereby providing an “intentional” wireless power transmission system for specific electrical devices. From the point of view of the energy harvesting circuit or device, however, the intentional EM radiation from the EM energy transmitter is the same or similar to the ambient (unintentional) energy, except that the intentional situation may result in a more reliable energy source. Both intentional and unintentional transmitted energy can be used for energy harvesting.
The harvested energy is generally captured for use or stored for future use by small, typically wireless, typically autonomous electronic circuits, components or devices, such as those used in some types of wearable electronics and wireless sensor devices or networks. Energy harvesting circuits or devices, thus, typically provide a very small amount of power for low-energy electronic circuits or devices electrically connected to, integrated with, or otherwise associated with the energy harvesting circuits or devices. These energy harvesting circuits are typically a supplemental power source to a battery on the device, as the EH sources do not provide sufficient power for the entire device or do not provide consistent power.
Antennas play an important role in the ability to harvest energy efficiently. The development of antennas for energy harvesting as well as for communication in wireless and IoT devices has involved studies to minimize size, increase efficiency, achieve multi-band frequencies, and investigate different antenna materials. Antennas have been incorporated into housings for mobile devices, into implantable devices, and onto smart cards and packaging. RFID antennas are often deposited onto the surfaces of labels for packaging or displays, such as small size peel-and-stick labels. Some antennas have been fabricated by printing—such as by silk-screening, flexographic, or ink-jet. Silver inks are the most commonly used ink for electrically conductive components, although carbon and polymer-based inks have also been used. As wireless devices become increasingly widespread, there is a continuing need for more efficient, cost-effective antennas.